The present invention relates to an antenna arrangement comprising a substantially planar patch conductor, and to a radio communications apparatus incorporating such an arrangement.
Wireless terminals, such as mobile phone handsets, typically incorporate either an external antenna, such as a normal mode helix or meander line antenna, or an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar.
Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrowband. However, cellular radio communication systems typically have a fractional bandwidth of 10% or more. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of a patch antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Further, PIFAs become reactive at resonance as the patch height is increased, which is necessary to improve bandwidth.
International patent application WO 01/37369 discloses a PIFA in which matching is achieved by linking feed and shorting pins with a conductive matching element whose dimensions are chosen to provide a suitable impedance match to the antenna. Such an antenna is inherently narrowband.
European patent application EP 0,867,967 discloses a PIFA in which the feed pin is meandered to increase its length, thereby increasing its inductance in an attempt to make the antenna easier to match. A broadband match is difficult to achieve with such an antenna, requiring a small matching capacitance.
Our co-pending unpublished International patent application PCT/IB02/00051 (Applicant""s reference PHGB 010009) discloses a variation on a conventional PIFA in which a slot is introduced in the PIFA between the feed pin and shorting pin. Such an arrangement provided an antenna having substantially improved impedance characteristics while requiring a smaller volume than a conventional PIFA.
An object of the present invention is to provide an improved planar antenna arrangement.
According to a first aspect of the present invention there is provided a antenna arrangement comprising a substantially planar patch conductor, a feed pin connected to the patch conductor at a first point and a ground pin connected between a second point on the patch conductor and a ground plane, wherein the arrangement further comprises a linking conductor connecting the feed and ground pins and shunt capacitance means coupled between the feed and ground pins, wherein the location and dimensions of the linking conductor and value of the capacitance means are selected to enable a good match to the antenna to be achieved.
The presence of the linking conductor acts to reduce the length of the short circuit transmission line formed by the feed and ground pins, and hence its inductance, enabling the value of the shunt capacitance to be increased which provides improved bandwidth. The linking conductor may also be connected to the patch conductor, or there may be gaps between the pins both above and below the linking conductor. By arranging for the matching inductance to be provided as part of the antenna structure, the inductance has a higher Q than that provided by circuit solutions at no additional cost.
The feed and ground pins may have different cross-sectional areas, to provide an impedance transformation. Alternatively, or in addition, one or both of the feed and ground pins may be formed of a plurality of conductors to provide an impedance transformation. The impedance transformation may also be provided by a slot in the patch conductor between the feed and ground pins, as disclosed in PCT/IB02/00051.
According to a second aspect of the present invention there is provided a radio communications apparatus including an antenna arrangement made in accordance with the present invention.